1. Field of the Invention
This invention relates generally to photoelectric conversion method and apparatus thereof for sensing light positions, and more particularly to such photoelectric apparatus as photoelectric switches for sensing the presence of objects optically, displacement sensors for sensing distances to objects by using optical reflection or for sensing displacements from preset points, and photoelectric sensors applicable to the displacement sensors or distance-determining type photoelectric switches.
2. Description of the Prior Art
Conventionally, direct reflection type photoelectric switches have been used as photoelectric conversion apparatus. They have been equipped; with a light-irradiating section and a light-receiving section on a common housing and have sensed the light irradiated from the light-irradiating section and reflected diffusively by the surface of objects to be sensed on the light-receiving section.
As the direct reflection type photoelectric switches can be operated in the state of being equipped in only one sensor housing and do not need such auxiliary parts as reflection plates, they have such advantages as being easily attached, adjusted and maintained, and cheap. Accordingly they have been used widely.
However, the direct reflection type photoelectric switches may have following problems, since they utilize the light scattered on objects.
That is:
(A) The sensible distance is shorter than other systems'.
(B) The sensible distances vary widely depending on the surface reflectances of objects to be sensed.
(C) In the case where the reflectance of a background is large, it is impossible to sense small reflectance objects being in front of the background.
A distance-determining type photoelectric switch composed for solving these problems, especially the problem (B) and (C), is disclosed in Japanese Patent Publication Syo 58-42411. Although this photoelectric switch is same as the direct reflection type photoelectric switch in the point that it irradiates light from its light-irradiating section to objects to be sensed and catches the light scattered on the objects on its light-receiving section, it is characterized in its light-sensing method in the light-receiving section.
FIG. 1 is a vertical section view showing the composition and operation of a light-position-sensing element 10 used in conventional photoelectric switches. FIG. 2 is a characteristic diagram explaining the characteristic of the light-position-sensing element 10 disclosed in FIG. 1. FIG. 3 is a diagram showing the photoelectric switch's construction drawn with single lines to explain the basic operation of the photoelectric switch for position-sensing as a photoelectric conversion apparatus. FIG. 4 is a characteristic diagram showing a relationship between distances X from the photoelectric switch to objects to be sensed and incident light positions Y, where bright spot images are formed, on a light-receiving surface.
In FIG. 3 above noted, "1" denotes a photoelectric switch. "2" denotes a lens system mainly composed of lenses, corresponding to the optical system. "3" denotes a light-irradiating section mainly composed of such a light-emitting element as a light-emitting diode. "4" denotes a light-receiving section disposed on a-light-receiving surface, mainly composed of light-receiving elements. The light-position-sensing element 10 is disposed on the light-receiving surface. "5", "6" and "7" respectively denote a signal processing section, objects to be sensed, and a background.
Now, in the case where incident light exists in the position where the light-position-sensing element 10 is located, two electric current outputs Ia and Ib are obtained from the light-position-sensing element 10. Then, the incident light position Y on the light-position-sensing element 10 is derived from the following equation: EQU (Ia-Ib)/(Ia+Ib)=2Y/L (1)
where "L" denotes the effective length of the light-position-sensing element 10.
The relationship of the equation (1) can be depicted as FIG. 2. As mentioned above, the incident light position Y on the light-position-sensing element 10 can be derived using electric signals. So the light-position-sensing element 10 has been used in photoelectric switches, as shown in FIG. 3.
Namely, the photoelectric switch 1 is equipped with the lens system 2, the light-irradiating section 3, the light-receiving section 4 and the signal-processing section 5, as shown in FIG. 3. Generally, such kinds of photoelectric switches are used for sensing the presence of objects to be sensed. For example, being disposed near a conveyor, they are used for sensing the presence of monitors equipped in computers conveyed by the conveyor.
Then, the light irradiated from the light-irradiating section 3 forms bright spots on the objects 6, i.e. 6A and 6B. Further, the light reflected by the objects 6 forms bright spot images on the light-receiving surface of the light-receiving section 4 through the lens system 2. In the case where the distances X to the objects 6 vary, the bright spot image positions Y move. Accordingly, determining the bright spot image positions Y on the light-receiving surface makes it possible to determine whether the bright spot images was formed by the objects 6 or by the background 7 placed on a farther point from the photoelectric switch 1 than the sensing objects 6 are located.
Consequently, the light-position-sensing element 10 shown in FIG. 1 is located on the light-receiving surface of the light-receiving section 4 for determining the bright spot image positions, i.e. the incident light positions Y. Then, the photoelectric switch generates sensing signals when the bright spot image positions Y are larger than a predetermined value. The photoelectric switches in this type have such a great deal of superior characteristics solving the above described problems of (B) and (C) compared to the switches of the type of sensing the quantity of light on the light-receiving surface that the sensible distance scarcely varies in spite of the variation of the surface reflectances of the objects to be sensed, and the objects can be sensed stably in the case where small reflectance objects to be sensed, e.g. black receptacles, flow between a large reflectance background 7, e.g. a white wall, and the photoelectric switch.
Next, the relationship between the distances X and the incident light positions Y is expressed by the next equation: EQU X * Y=L2 * L3
where L2 denotes optical axis pitches between projected lights and received lights, and L3 denotes the lens lengths of the light-receiving elements. It is FIG. 4 that depicts this X-Y relationship. Thus, the light-position-sensing element 10 can derive the incident light positions Y electrically, and then the distances X can be obtained. Therefore it has been widely used in displacement sensors outputting differences from a reference distance as displacements.
Because conventional photoelectric conversion methods and apparatus are composed as above mentioned, the relationships between the distances X to the objects 6 and the bright spot image positions Y on the light-receiving surface are nonlinear, as shown in FIG. 4, in the case where the light-position-sensing elements are applied to the photoelectric switches. That is, when the distances X to the objects 6 are small, the variations of the bright spot image positions Y on the light-receiving surface are large compared to the distances X to the objects 6. Then the variation amount of two output currents Ia and Ib of the light-position-sensing element 10 is large, so the bright spot image positions Y can be operated precisely, and the distances X to the objects 6 can be determined precisely. However, the variations of the bright spot image positions Y are small compared to the distances X to the objects 6 in the case where the distances X to the objects 6 are large. Then the variation amount of two output currents Ia and Ib of the light-position-sensing element 10 is small, so the bright spot image positions Y cannot be operated precisely and the distances X to the objects 6 cannot determine precisely. Though this situation can be improved by enlarging the distances from the light-irradiating axis to the light-receiving section 4, it brings about such a defect that the sizes of photoelectric switches are enlarged. Further the brightness of the bright spots on the light-receiving surface becomes dark in proportion to to the distances X to the objects 6, then the influence of noises comes to be large and the accuracy of the determination of distances comes to be bad. So, it is impossible to lengthen the measurable distance of the photoelectric switch.
Also, the application of the light-position-sensing element 10 to displacement sensors makes X * Y=L2 * L3 large, and improves its distance measurement sensitivity, however, not only the problem that the size of the sensor comes to large happens, but the problems as will be described next are caused by the application.
That is, the amount of variations of Y must not be exceeding L in the case where the effective length L of the light-receiving surface is assumed to be constant, because Y varies much compared to X. Then the span of X comes to be very narrow, even though the setting of the reference distance is made to be variable. Therefore, even if the light-position-sensing element 10 is effective for such a fine adjustment as the correction of errors of the reference distance, it has such problems that the reference distances is not changeable.